Finance group McMillan Shakespeare's share price has almost halved after it said the Rudd government's flagged changes to fringe benefit tax (FBT) laws had created uncertainty.

After emerging from a week-long trading suspension, its share price crashed by 48.05%, or $7.37, to $7.99, wiping $549m from its market value.

McMillan Shakespeare's request for an extension to a trading halt was rejected by the Australian Securities and Investments Commission.

In a statement, it criticised the government's decision last week to tighten FBT guidelines on car leasing and salary-sacrifice packaging.

"The proposed changes, which abolish the 28-year-old practice of being able to rely on the statutory formula method to quantify the amount of FBT payable on employer-provided motor vehicles ... is creating disruption within the industry and is expected to lead to an unknown and unquantifiable decrease in demand for novated leases and an adverse impact to the business overall," it said.

On its website, McMillan describes itself as a provider of salary packaging and vehicle leasing administration.

While the treasurer, Chris Bowen's proposals are yet to pass parliament, McMillan said the changes would have a "material adverse impact" on the company's future earnings.

Chief financial officer Mark Blackburn said McMillan would suspend all communication with investment analysts, shareholders and the media until after the election "unless the position becomes clearer prior to then".

McMillan is expecting a net profit of between $61m and $63m for fiscal 2013, representing a 15% increase on the prior financial year.